 CKD subsystems and directors were offered by IBM and plug compatible competitors until at least 1996 (2301 to 3390 Model 9); in total 22 unique DASD offered by IBM configured in at least 35 different subsystem configurations.Plug-compatible offered many of the same DASD including 4 CKD subsystems featuring unique DASD.The initial feature set provided by IBM with its 1964 introduction of the CKD track format and associated CCWs included